Correlation of Molecular Structure with Photophysical Properties and

Jan 28, 2016 - emitting core based TADF emitter derived from a 3,3′-bicarbazole donor ..... This work was supported by Samsung SDI, development of r...
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Correlation of Molecular Structure with Photophysical Properties and Device Performances of Thermally Activated Delayed Fluorescent Emitters Mounggon Kim,† Sang Kyu Jeon,‡ Seok-Ho Hwang,† Sang-Shin Lee,§ Eunsun Yu,§ and Jun Yeob Lee*,‡ †

Department of Polymer Science and Engineering, Dankook University, 152 Jukjoen-ro, Suji-gu, Yongin, Gyeonggi 448-701, Korea School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi 440-746, Korea § Central R&D Center, Samsung SDI, Samsung-Ro 130, Yeongtong-Gu, Suwon, Gyeonggi 443-803, Korea ‡

ABSTRACT: Thermally activated delayed fluorescent (TADF) emitters having bicarbazole as a donor unit based on a twin emitting core design were developed by substituting the bicarbazole donor with a diphenyltriazine acceptor. Three bicarbazole derived delayed fluorescent emitters linked via 2,3′-, 3,3′-, and 3,4′-positions of carbazole were synthesized and investigated as blue dopants of the TADF devices. A twin emitting core based TADF emitter derived from a 3,3′-bicarbazole donor exhibited a high quantum efficiency of 25.0% in the greenish blue TADF device. A 3,3′-linkage induced red shift of the emission wavelength, a small singlet−triplet energy gap, and a high quantum efficiency were revealed.

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carbazole,1,2,8,11,23,24 and benzofurocarbazole25 have been used as electron donors of the TADF emitters. However, further study to engineer the donor moiety and core structure for highefficiency TADF emitters is needed to enhance the device performances of the TADF devices. In this work, twin emitting core based TADF emitters derived from a bicarbazole donor moiety and a diphenyltriazine acceptor moiety were synthesized, and the effect of the bicarbazole donor on the photophysical properties and device characteristics of the TADF emitters was studied. Three different bicarbazole donors, 2,3′-bicarbazole, 3,3′-bicarbazole, and 3,4′-bicarbazole, were combined with a diphenyltriazine acceptor in a twin emitting core type backbone structure. It was revealed that the 3,3′-bicarbazole moiety the reduced singlet− triplet energy gap (ΔEST) and enhanced the quantum efficiency of the TADF devices due to strong electron-donating power induced by extended conjugation. A high quantum efficiency of 25.0% was obtained using a 3,3′-bicarbazole and diphenyltriazine derived TADF emitter.

INTRODUCTION In general, thermally activated delayed fluorescent (TADF) emitters consist of donor and acceptor moieties linked via an aromatic linking unit.1−19 The type of donor and acceptor moieties is critical to the light-emitting properties of the TADF emitters, and proper selection of the donor and acceptor units is important for the design of the TADF emitters. The most widely used donor moieties of TADF emitters are carbazole and acridine, which are known as electron-donating moieties. Carbazole has less electron-donating character than acridine and can be efficient as the donor moiety of the TADF emitters with a strong electron-acceptor unit. Sometimes, several carbazole moieties can be included in one molecule to strengthen the donor character of the TADF emitters. For example, a well-known green TADF emitter, 4CzIPN, was designed to have four carbazole units in the molecular structure for strong donor character.1 Other than these, several TADF emitters with the carbazole donors were reported.2,5,9,11,12,14,17,18 However, the carbazole moiety is a weak electron donor, and strong electron donors are required to adjust the emission wavelength of the TADF emitters. Acridine was popular as a relatively strong electron-donor moiety.7,20−22 It can be merged into a molecular structure of a TADF emitter with both strong and weak electron-accepting units due to its strong electron-donating character, which renders the acridine merged molecule to have TADF character. A well-known blue TADF emitter, bis[4-(9,9-dimethyl-9,10dihydroacridine)phenyl]sulfone, had the acridine moiety and a weak electron-accepting sulfone moiety.7 Other than these, phenoxazine,3,4,6,7,9 phenothiazine,19 alkyl- or alkoxy-modified © 2016 American Chemical Society

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EXPERIMENTAL SECTION General Information. Potassium carbonate, potassium phosphate, and 1,4-dioxane were purchased from Duksan Sci. Co. Bis(pinacolato)diboron was purchased from P&H Co., and 2-chloro-4,6-diphenyl-1,3,5-triazine was obtained from Sun Fine Global Co. Copper iodide, tetrakis(triphenylphosphine)-

Received: September 18, 2015 Revised: January 18, 2016 Published: January 28, 2016 2485

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2,3′-bicarbazole (0.15 g, 0.5 mmol) was used instead of 9H,9′H-2,3′-bicarbazole. The product was obtained as a powder (0.19 g, yield 45%). 1H NMR (400 MHz, CDCl3): δ 9.11−9.06 (m, 4H), 8.86−8.83 (m, 8H), 8.48 (s, 1H), 8.19 (d, 2H, J = 7.6 Hz), 7.96 (d, 2H, J = 8.4 Hz), 7.87 (d, 2H, J = 6.8 Hz), 7.81−7.74 (m, 2H), 7.68−7.49 (m, 18H), 7.39−7.33 (m, 3H), 7.02 (t, 1H, J = 14.8 Hz). Tg, 200 °C. MS (FAB) m/z 948[(M+H)+]. Anal. Calcd for C66 H42 N8: C, 83.70; H, 4.47; N, 11.83. Found: C, 84.21; H, 4.29; N, 11.80. Device Fabrication. All devices reported in this work were fabricated using a thermal evaporator with a vacuum pressure of 1.0 × 10−6 Torr and grown on a 120 nm thick indium tin oxide substrate by coating poly(3,4-ethylenedioxythiophene):poly( s t y r e n e s u lf o n a t e ) ( P E D O T : P SS , 6 0 n m ) , 4 , 4′ cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline] (TAPC, 10 nm), 1,3-bis(N-carbazolyl)benzene (mCP, 10 nm), an emitting layer, diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 5 nm), and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI, 30 nm) organic materials step by step. LiF (1 nm) and Al (200 nm) were deposited on the TPBI layer by vacuum thermal evaporation. The emitting layers were 25 nm thick bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO):23TCzTTrz, DPEPO:33TCzTTrz, and DPEPO:34TCzTTrz, with the TADF emitters at doping concentrations of 10, 20, and 30%. Measurements and Characterization. The ionization potential (IP) and electron affinity (EA) were calculated from oxidation and reduction curves obtained using a carbon working electrode, Pt wire counter electrode, and Ag/AgCl reference electrode. The IP was calculated from the edge of oxidation curves, and the voltage range was from 0 to 2 V. The EA was calculated from the edge of reduction curves, and the voltage range was from 0 to −2 V. The oxidation and reduction scans were carried out separately. Bu4NPF6 dissolved in methylene dichloride for oxidation and 0.1 M BuNClO4 dissolved in THF for reduction was an electrolyte for the electrochemical characterization. The singlet and triplet energy was estimated from the onset of the fluorescent and phosphorescent spectra collected using a 1.0 × 10−5 M toluene solution. The phosphorescent spectra were measured at 77 K. The delayed lifetime was calculated from room-temperature transient photoluminescence (PL) decay curves of the vacuumdeposited films. Absolute PL quantum yield data were gathered using an integrating sphere using the same vacuum-deposited films. A Keithley 2400 source measurement was used for electrical characterization of the fabricated devices by the voltage sweep method, and a CS 2000 spectroradiometer was used to measure the light emission characteristics. The external quantum efficiency (EQE) was calculated based on an assumption of the Lambertian emission distribution.

palladium(0), 4-bromophenylboronic acid, and (±)-trans-1,2diaminocyclohexane were obtained from Aldrich. Co. 9H,9′H3,3′-bicarbazole and 9H,9′H-3,4′-bicarbazole were synthesized according to the literature.26 2-(4-Bromophenyl)-4,6-diphenyl-1,3,5-triazine (A). 2Chloro-4,6-diphenyl-1,3,5-triazine (5 g, 18.6 mmol) and (4bromophenyl)boronic acid (4.88 g, 24.3 mmol) were dissolved in tetrahydrofuran (THF, 80 mL). Potassium carbonate solution (40 mL, 2 M) and tetrakis(triphenylphosphine)palladium(0) (0.65 g, 0.6 mmol) were added to the solution, and the solution was stirred for 3 h followed by filtering of residual product. The product was washed with dichloromethane, hexane, and distilled water followed by purification again by sublimation. The product yield was 3.7 g (yield 51%). 1 H NMR (400 MHz, CDCl3): δ 8.75 (d, 4H, J = 6.8 Hz), 8.64 (d, 2H, J = 7.2 Hz), 7.71 (d, 2H, J = 7.2 Hz), 7.61−7.57 (m, 6H). MS (FAB) m/z 389[(M+H)+]. 9H,9′H-2,3′-Bicarbazole. 2-Bromo-9H-carbazole (1.6 g, 6.1 mmol), 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9Hcarbazole (2.48 g, 8.5 mmol), and tetrakis(triphenylphosphine)palladium(0) (0.45 g, 0.4 mmol) were dissolved in THF (60 mL) followed by addition of 2 M potassium carbonate solution (30 mL). The mixture was refluxed for 24 h and cooled to room temperature. The residue was filtered and washed with hexane and ethyl acetate. The product yield was 1.41 g (yield 44%). 1H NMR (400 MHz, CDCl3): δ 8.39 (s, 1H), 8.15 (d, 2H, J = 8.4 Hz), 8.10 (d, 1H, J = 7.2 Hz), 7.78 (d, 1H, J = 8.8hz), 7.74 (s, 1H), 7.61 (d, 1H, J = 8.0 Hz), 7.53 (d, 1H, J = 8.4 Hz), 7.47−7.40 (m, 4H), 7.29−7.24 (m, 4H). MS (FAB) m/z 332[(M+H)+]. 9,9′-Bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)9H,9′H-2,3′-bicarbazole (23TCzTTrz). 2-(4-Bromophenyl)4,6-diphenyl-1,3,5-triazine (0.67g, 1.7 mmol), 9H,9′H-2,3′bicarbazole (0.25g, 0.8 mmol), copper iodide (0.17g, 0.9 mmol), and potassium phosphate (0.96g, 4.5 mmol) were vacuum-dried for 2h, and (±)-trans-1,2-diamino cyclohexane (0.10 g, 0.9 mmol) in 1,4-dioxane (40 mL) was added to the mixture. The mixture was stirred for 12 h and cooled to room temperature. The mixture was filtered to obtain residual product. The product was washed using ethyl acetate, dichloromethane, and hexane and purified again by sublimation. The product was obtained as a powder (0.48 g, yield 68%). 1H NMR (400 MHz, CDCl3): δ 9.07 (d, 2H, J = 8.8 Hz), 9.03 (d, 2H, J = 8.0 Hz), 8.84−8.81 (m, 8H), 8.43 (s, 1H), 8.28−8.20 (m, 3H), 7.91 (d, 2H, J = 8.8 Hz), 7.87−7.83 (m, 3H), 7.79− 7.73 (m, 2H), 7.66−7.31 (m, 19H). Tg, 193 °C. MS (FAB) m/z 948[(M+H)+]. Anal. Calcd for C66 H42 N8: C, 83.70; H, 4.47; N, 11.83. Found: C, 83.60; H, 4.37; N, 11.35. 9,9′-Bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)9H,9′H-3,3′-bicarbazole (33TCzTTrz). 33TCzTTrz was synthesized by the same procedure as 23TCzTTrz. 9H,9′H3,3′-bicarbazole (0.28 g, 0.8 mmol) was used instead of 9H,9′H-2,3′-bicarbazole. The product was obtained as a powder (0.6 g, yield 79%). 1H NMR (400 MHz, CDCl3): δ 9.07 (d, 4H, J = 8.8 Hz), 8.84 (d, 8H, J = 8.4 Hz), 8.51 (s, 2H), 8.28 (d, 2H, J = 7.6 Hz), 7.90−7.84 (m, 6H), 7.69−7.60 (m, 16H), 7.49 (t, 2H, J = 15.6 Hz), 7.38 (t, 2H, J = 15.2 Hz). Tg, 205 °C. MS (FAB) m/z 948[(M+H)+]. Anal. Calcd for C66 H42 N8: C, 83.70; H, 4.47; N, 11.83. Found: C, 83.36; H, 4.14; N, 11.30. 9,9′-Bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)9H,9′H-3,4′-bicarbazole (34TCzTTrz). 34TCzTTrz was synthesized by the same procedure as 22TCzTTrz. 9H,9′H-

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RESULTS AND DISCUSSION The three TADF emitters developed in this work had bicarbazole as a donor moiety and two diphenyltriazine acceptor moieties linked to the bicarbazole moiety through a phenyl spacer to reduce the ΔEST and to realize high quantum efficiency in the TADF devices. The two diphenyltriazine acceptor moieties may allow the TADF emitters to behave as a twin emitter with two TADF emitter units. Therefore, the 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz emitters may have strong light absorption properties and light emission properties. Additionally, the three TADF emitters were designed to study 2486

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The Journal of Physical Chemistry C Scheme 1. Synthesis of Bicarbazole Donor Based TADF Emittersa

a Conditions: (a) Pd(0), K2CO3, THF, reflux 24 h. (b) CH3COOK, 1,4-dioxane, reflux, 48 h. (c) CuI, K3PO4, trans-1,2-diamino cyclohexane, 1,4dioxane, reflux, 12 h.

the effect of interconnect position of bicarbazole on the physical and optical properties of the TADF emitters. The synthetic scheme of TADF materials is schematically presented in Scheme 1. Three different bicarbazole intermediates, 2,3′-bicarbazole, 3,3′-bicarbazole, and 3,4′-bicarbazole, were prepared by a coupling reaction, and then, they were coupled with a brominated triphenyltriazine. Highly pure TADF materials purified by vacuum train sublimation were used for material characterization and device fabrication. Physical and optical characterization of the TADF materials is essential to correlate the molecular structure of the TADF materials with light absorption and emission properties. Ultraviolet−visible (UV−vis) and PL characterization results of the TADF materials are compared in Figures 1 and 2. UV− vis absorption spectra measured using a toluene solution of the TADF emitter proved that the combination of two TADF emitter units intensified UV−vis absorption and the 3,3′-

bicarbazole core extended the absorption peak to long wavelength. The high absorption coefficient of the three TADF emitters originated from enhanced absorption by two TADF emitter units included in the TADF emitters. Compared with the UV−vis absorption of 9-(4-(4,6-diphenyl-1,3,5-triazin2-yl)phenyl)-9H-carbazole (PhCzTrz), that of the three TADF emitters was dramatically increased by the twin emitter type molecular design. In addition to the strong absorption, the UV−vis absorption spectra were extended to long wavelength in 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz. The red shift of the absorption edge from 381 nm of PhCzTrz to 403 nm (23TCzTTrz), 422 nm (33TCzTTrz), and 403 nm (34TCzTTrz) indicates that the three TADF emitters have relatively stronger donor units than PhCzTrz. This result can be predictable because the bicarbazole unit may have extended conjugation and strengthened donor character by a combination of two donor units. The less pronounced red shift of the 2487

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because of strong electron-donating character of 3,3′bicarbazole. The increased donor character of 3,3′-bicarbazole intensified the CT character and lowered the singlet energy of 33TCzTTrz. The order of triplet energies estimated from the onset of the phosphorescent spectra was 34TCzTTrz (2.79 eV) > 33TCzTTrz (2.76 eV) > 23TCzTTrz (2.75 eV). Therefore, the ΔEST values of 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz were 0.33, 0.25, and 0.35 eV, respectively. The strong electrondonating character of 3,3′-bicarbazole decreased the ΔEST of 33TCzTTrz, which implies that 33TCzTrz may be better than other materials to exhibit TADF characteristics. As the molecular orbital distribution of 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz is important to understand photophysical properties of the TADF materials, the HOMO and the LUMO of the three TADF materials were studied. The HOMO and LUMO distribution of three TADF materials obtained by Gaussian theoretical calculation using the B3LYP/ 6-31G(d) function is displayed in Figure 3. In all compounds, the HOMO was similarly found in the bicarbazole donor moiety, the LUMO was located on the diphenyltriazine moiety, and all TADF materials were designed to have donor−acceptor type molecular structure. The HOMO and LUMO were largely separated with weak overlap at the phenyl linkage connecting the bicarbazole donor and diphenyltriazine acceptor. Therefore, the 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz emitters fulfilled the molecular orbital requirement of TADF emitters. Although the molecular orbital distribution was similar, the dihedral angle between two carbazole units was dependent on the linking positions. The dihedral angle of 23TCzTTrz and 33TCzTTrz was 38°, but it was 58° in 34TCzTTrz. The two carbazole units of bicarbazole were distorted from each other in the 34TCzTTrz by steric hindrance. As explained above, the large dihedral angle increased the singlet energy and triplet energy of 34TCzTTrz. The IP and EA were measured by cyclic voltammetry (CV) to predict the HOMO and LUMO of the TADF emitters. CV scan data of the TADF emitters are shown in Figure 4. The IP from the onset of oxidation scans became shallow in 33TCzTTrz, and the EA from the onset of reduction scans became deep in 34TCzTTrz. The IP was strongly affected by the donor moiety, and the strong electron-donating 3,3′bicarbazole moiety shifted the IP toward vacuum level. The IP and EA of three TADF emitters are summarized in Table 1 in addition to other photophysical properties. As 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz were developed as TADF emitters, delayed fluorescent emission of the three emitters was studied by investigating transient PL

Figure 1. UV−vis absorption spectra of PhCzTrz and three TADF emitters.

absorption edge of 34TCzTTrz may be caused by a large dihedral angle between two carbazole units, which limits the extension of conjugation between two carbazole units and decreases the donor character, as shown in Figure 3. The relatively small red shift of the absorption edge of 23TCzTTrz is due to the fact that the 2-position of carbazole is a node for the HOMO. As the 2-position of carbazole is a node position for the HOMO, the connection through the 2-position of carbazole spatially separates the HOMO between two carbazole units. Therefore, the degree of conjugation and the donor strength of 2,3′-bicarbazole are not increased by linking two carbazole units, and the small red shift of the UV−vis absorption edge was observed in the 23TCzTTrz emitter. Fluorescent and phosphorescent PL spectra of the TADF emitters in Figure 2 followed the trend of the UV−vis absorption edge. The fluorescent spectra were obtained at room temperature, and the phosphorescent spectra were gathered at 77 K in liquid nitrogen after a delay time of 10 μs to the remove fluorescent emission component. A 1.0 × 10−5 M THF solution was the sample for both fluorescent and phosphorescent emission characterization. Solution PL emission in toluene was caused by charge transfer (CT) emission of the TADF emitters, and singlet energies of 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz were 3.08, 3.01, and 3.14 eV, respectively, from the onset of the fluorescent emission. Compared to the singlet energy of 23TCzTTrz and 34TCzTTrz, the singlet energy of 33TCzTTrz was lowered

Figure 2. Solution (a) and low-temperature (b) PL spectra of the TADF emitters. 2488

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Figure 3. Molecule orbital distribution of the HOMO and LUMO of TADF emitters.

Figure 4. CV curves of TADF emitters.

compared to that of other TADF emitters resulted in the short excited-state lifetime. On the basis of the material characterization data, TADF devices were developed using DPEPO as the host material. The device structure and energy level diagram of the TADF devices are presented in Figure 6. In order to confine all excitons in the emitting layer, high triplet energy mCP and TSPO1 were used

decay. Figure 5 shows transient PL decay curves of 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz. The delayed PL emission was clearly detected in 33TCzTTrz due to a small ΔEST, but it was weakly observed in 34TCzTTrz due to a large ΔEST. Excited-state lifetimes for delayed emission were 14.1, 8.4, and 20.2 μs for 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz, respectively. The relatively small ΔEST of 33TCzTTrz 2489

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The Journal of Physical Chemistry C Table 1. Photophysical and Electrochemical Properties of Bicarbazole Donor-Based TADF Emitters emitters

BGa (eV)

IP (eV)

EA (eV)

S1 (eV)

T1 (eV)

ΔEST (eV)

εb

τc (μs)

τd (ns)

ΦPLe (%)

ΦPLf (%)

23TCzTTrz 33TCzTTrz 34TCzTTrz

3.01 2.94 3.06

−5.86 −5.74 −5.98

−3.19 −3.21 −3.19

3.08 3.01 3.14

2.75 2.76 2.79

0.33 0.25 0.35

1.5 1.7 1.7

14.1 8.4 20.2

32.8 46.0 31.5

82 87 65

83(74) 100(21) 65(53)

The band gap was calculated from the edge of the absorption spectrum. bThe absorption coefficient was measured in THF solution (105 M−1 cm1−). cDelayed PL lifetime of TADF materials. dPrompt PL lifetime of TADF materials. eQuantum yield of TADF materials in THF under nitrogen bubbling. fQuantum yield of 20% doped TADF materials in a DPEPO matrix with and without (in parentheses) nitrogen bubbling. a

Figure 5. Transient PL decay curves of (a) delayed and (b) prompt components for TADF emitters.

Figure 6. Device structure and energy level diagram of the TADF devices.

efficiency of 4.2% of carbazole and diphenyltriazine based PhCzTrz emitters, the 33TCzTTrz device exhibited a dramatically improved quantum efficiency of 25.0%. There can be several material parameters to be considered to explain the high quantum efficiency of the 33TCzTTrz devices. One important parameter is the high PL quantum yield of 33TCzTTrz. The PL quantum yield of the 33TCzTTrz doped DPEPO film under oxygen was 21%, but it was greatly increased to 100% under nitrogen, which indicates that triplet excitons mainly contribute to the PL emission of 33TCzTTrz by a reverse intersystem crossing process. All triplet excitons

as a thin layer to prevent exciton quenching by CT layers. Optimized quantum efficiency data of the TADF devices were plotted against luminance in Figure 7. Current density, voltage, and luminance data of the TADF devices are summarized in Figure 8 according to the doping concentration of the TADF emitters. Maximum quantum efficiencies of the optimized 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz devices were 10.7, 25.0, and 10.3%, respectively. 33TCzTTrz showed the best quantum efficiency among all TADF emitters developed in this work and displayed one of the best quantum efficiencies reported in greenish blue devices.4,22 Despite the low quantum 2490

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33TCzTTrz, the quantum efficiency was more than doubled compared to the PhCzTrz emitter, mostly owing to strong light absorption and the slightly intense electron-donating character of 2,3′-bicarbazole and 3,4′-bicarbazole, which gave rise to delayed fluorescent emission in 23TCzTTrz and 34TCzTTrz. Figure 9 represents electroluminescence (EL) spectra of the TADF devices. The order of peak position of the TADF

Figure 7. Optimized quantum efficiency curves of the TADF devices.

were converted into singlet excitons completely in the PL emission process. The efficient reverse intersystem crossing process by small ΔEST increased the quantum efficiency of the 33TCzTTrz by harvesting triplet excitons for radiative transition. Another important parameter is the shallow HOMO level of 33TCzTTrz. The HOMO level estimated from the IP of the TADF emitters was shallow in 33TCzTTrz, which could facilitate hole injection from mCP to the emitting layer. As reported in other work,27 the DPEPO host cannot inject holes due to the deep HOMO level of −6.80 eV, which breaks charge balance in the emitting layer. Therefore, the charge balance in the DPEPO host based emitting layer can be improved using an emitting material that can inject holes efficiently, and 33TCzTTrz was superior to 23TCzTTrz and 34TCzTTrz. Although the quantum efficiency of the 23TCzTTrz and 34TCzTTrz devices was lower than that of

Figure 9. EL spectra of the TADF devices.

devices was 33TCzTTrz (490 nm) > 23TCzTTrz (472 nm) > 34TCzTTrz (463 nm), which agreed with the order of the PL emission peaks of the TADF emitters even though the EL emission peaks were relatively shifted to long wavelength. The shift of the EL emission peaks is caused by a strong intermolecular interaction between emitters in solid films and a strong polar−polar interaction between the DPEPO host and TADF emitters that originated from the high polarity of the DPEPO host. Color coordinates of 23TCzTTrz, 33TCzTTrz, and 34TCzTTrz devices were (0.20,0.26), (0.23,0.42), and

Figure 8. Current density, voltage, and luminance characteristics of the TADF devices according to the doping concentration. 2491

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The Journal of Physical Chemistry C Table 2. Summarized Device Performances of the TADF Devices devices

doping concentration (%)

23TCzTTrz

10 20 30 10 20 30 10 20 30

33TCzTTrz

34TCzTTrz

color coordinate (x, y) max quantum efficiency (%) (0.18, (0.20, (0.20, (0.20, (0.23, (025, (0.15, (0.16, (0.22,

0.22) 0.26) 0.29) 0.33) 0.42) 0.47) 0.14) 0.20) 0.35)

10.5 10.7 9.2 20.6 25.0 21.1 9.0 10.3 6.1

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CONCLUSIONS Three twin emitting core based TADF emitters derived from a bicarbazole donor and a diphenyltriazine acceptor were synthesized, and the interconnect position of bicarbazole was of crucial importance to the TADF behavior of the emitting materials. 3,3′-Bicarbazole was better than 2,3′-bicarbazole and 3,4′-bicarbazole to reduce ΔEST of the TADF emitters and to induce delayed fluorescence emission. The 33TCzTTrz TADF emitter showed one of the best quantum efficiencies of 25.0% in the greenish blue TADF device. Therefore, the bicarbazole donor and twin emitter type molecular structure can be useful to develop highly efficient TADF emitters. AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 82-31-299-4716. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Samsung SDI, development of red and blue OLEDs with external quantum efficiency over 20% using delayed fluorescent materials funded by MOTIE, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT, and future Planning (2013R1A2A2A01067447).

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max power efficiency (lm/W)

21.2 28.4 24.1 45.7 64.3 57.3 14.7 19.1 16.3

16.6 22.3 19.0 39.1 57.7 46.9 11.6 15.0 12.8

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(0.16,0.20), respectively, which were dominated by the electron donor strength of the bicarbazole moiety. All device performances are presented in Table 2.

■

max current efficiency (cd/A)

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